Looking at Gamma Rays

The gamma-ray imaging spectrometer (GRIS) we have assembled comprises four coaligned, independent imagers, each with its own detector and coded-aperture mask (Figure 1). Each detector “sees” incoming gamma rays only through its mask, which serves as the imaging optic for the gamma rays (see box, pp. 1819). This mask is mounted on a movable mask plate in front of the detector plane; moving the plate provides different levels of zoom for the gamma-ray images. At the back of the housing are the electronics that take the relatively weak signals from the detectors and amplify them before they are sent to the data-acquisition system, which can be located remotely. Our system currently consists of a commercial electronics module, whose data are read out by a notebook computer (Figure 1). Coaligned with the gammaray imagers is a video camera. Images from this provide both a visual aim point and visible light images that can be overlaid with the gamma-ray images to pinpoint the location of the radioactive material.